Recording of reciprocal of temperature

ABSTRACT

Direct recording of the relationship between a measured property and the reciprocal of absolute temperature is afforded in viscosity and vapor pressure measurements by means of resistance thermometry in cooperation with a bridge circuit.

United States Patent 1191 Fowler et al.

RECORDING OF RECIPROCAL OF TEMPERATURE Inventors:

Assignee:

Filed:

App]. No.:

US. Cl. 73/l5.4, 73/25, 73/54, 73/343.5, 73/362 AR Int. Cl. G01n 25/00,GOlk 3/00 Field of Search 73/362 AR, 15 B,

Lewis Fowler; Walter N. Trump, both of St. Louis, Mo.

Monsanto Corporation, St. Louis, Mo.

Dec. 28, 1970 References Cited UNITED STATES PATENTS Hall 73/362 AR UXKeinath 73]] F UX Aug. 21, 1973 2,741,126 4/195 Anderson 6! al 73/362 ARx 3,076,955 2/1963 Huddleston 73/362 AR ux 3,182,507 5/1965 Rogen 73/362AR 3,283,560 11/1966 Harden et al. 73/15 B 2,727,968 12/1955 Rittner etal. 73/362 AR X 3,339,398 9/1967 Barrall 6161. 73/15 13 3,369,390 2/1968Chu 6161. 73/15.6 3,550,427 12/1970 Sueyoshi 73/15.6

Primary Examiner-Richard C. Queisser Assistant Examiner-Frederick ShoonAtt0rneyNeal E. Willis, J. E. Maurer and William H. Duffey 5 7] ABSTRACTDirect recording of the relationship between a measured property and thereciprocal of absolute temperature is afforded in viscosity and vaporpressure measurements by means of resistance thermometry in cooperationwith a bridge circuit.

5 Claims, 1 Drawing Figure CONVERTER x E OUT X-Y RECORDER VERT.

. HORIZ.

PMENIEB AUG? I 873 OVEN PRESSURE TRANSDUCER TEMPERATURE TRANSDUCER CELLAMPLIFIER FIXED RESISTQRS LOGARITHMIC CONVERTER X- Y RECORDER VERT.

TLos 9 HORIZ.

RECORDING OF RECIPROCALOF TEMPERATURE The present invention relates to amethod for direct recording of certain physical variables as a functionof inverse temperature, i.e., reciprocal of temperature.

Vapor pressure and viscosity are typical examples of variables adaptedto the teachings of the present invention since they tend to be linearfunctions of reciprocal of absolute temperature.

One of the tests used in development of synthetic lubricants andhydraulic fluids for high temperature service is the measurement ofvapor pressure as a function of fluid temperature. The vapor pressuremay be measured directly to give an indication of the rate ofevaporation loss to be expected. Alternatively, the rate of rise ofapparent vapor pressure in a closed space may be measured as anindication of decomposition rate.

Prior to the advancement taught by the present invention, the knownmethods for vapor pressure measurements of the type described above weremanual, usually employing an isoteniscope. The isoteniscope is aninstrument for the static determination of vapor pressure from thechange in level of a liquid in a U tube. A salt bath is usually employedfor heating and constant operator attention is required to measure vaporpressure by balancing against a measured pressure of nitrogen.

Viscosity is another variable applicable to the present teachings. Widerange recording of viscosity as a function of temperature istraditionally employed for evaluation of lubricants, functional fluids,lubricant additives and the like, and for general experimental viscositymeasurements.

No complete theory has been developed on the relationship betweenviscosity of a liquid and liquid temperature but there are two majortheories which have been applied, viz., the relaxation time theory andthe activation energy theory. Each of these theories, however, relatesto a limiting relationship in which the logarithm of viscosity is alinear function of the reciprocal of the absolute liquid temperature.Notwithstanding the fact that experimental viscosity data tend todeviate from this relationship, the deviation becoming more pronouncedwith increasing viscosity, the logarithm viscosity-reciprocal absolutetemperature function provides a useful basis for plotting andcorrelating the viscosity data.

A long-standing need has prevailed for a method of recording suchvariables as vapor pressure and viscosity as a function of reciprocal oftemperature, particularly reciprocal of absolute temperature. Prior artmethods for recording viscosity, even though complex and expensive, areat best capable of plotting viscosity as a function of actualtemperature.

The remarkable achievement afiorded by the present invention wasaccomplished by a novel application of resistance thermometry to providethe inverse temperature function. Surprisingly and unexpectedly, by adetermined choice of resistance values, the desired temperaturerelationship was achieved. It was thus discovered herein that areciprocal temperature scale could be provided on a conventional linearmilli'voltage x-y recorder by use of a resistance temperature transducerin a suitable bridge circuit.

It is an object of the present invention, therefore, to provide a methodfor recording physical data as a linear function of reciprocal oftemperature. Another object of the present invehtion is to affordrecording of reciprocal of temperature with simplified electronicequipment. Yet another object of the present invention isto attain aninverse temperature relationship through use of a resistance thermometerand a Wheatstone bridge wherein the resistance values are'ofdeterminedquantity and have a determined relationship. Other aspects, objects andadvantages of thisinvention will become apparent from a consideration ofthe accompanying disclosure, drawing and appended claims.

The sole FIGURE of the drawing is a schematic diagram of a preferredembodiment which utilizes the method of the parent invention.

In the method of the present invention for recording reciprocal oftemperature in the measurement of vapor pressure, viscosity and thelike, a resistance thermometer is used in cooperation with a suitablebridge circuit. While resistance thermometer-bridge combinations areknown in the prior art for measurement of temperature, the use ofspecialized bridge forms in such applications has heretofore been forthe purpose of obtaining an output voltage proportional to actualtemperature, not proportional to the reciprocal of temperature.

A useful working circuit for the purposes of the present invention isthe well known Wheatstone bridge. The Wheatstone bridge is fundamentallya device used to measure the electrical resistance of an unknownresistor by comparing the resistor with a known standard resistance. TheWheatstone bridge circuit is -a fourterminal network comprising fourresistors interconnected in a square layout, to the input of which isapplied a voltage. In the conventional application of the Wheatstonebridge, the values of three of the resistances are known and the valueof the fourth resistance is determined by comparison with the knownresistances.

In a preferred embodiment of the present invention, a platinumresistance temperature transducer of con ventional design is connectedto a direct current Wheatstone bridge. The bridge input voltage isidenti' tied herein as E,,,. The output voltage of the bridge,designated herein as E can be applied to the temperature axis of aconventional temperature recorder. The temperature transducer thusbecomes one of the four resistance components of the Wheatstone bridge,its resistance being identified herein as R This temperature transduceris the sensing device which senses the temperature to which the testfluid is exposed during measurements of fluid vapor pressure or fluidviscosity.

It has now been discovered that, by a suitable choice of resistance inthe bridge arm in series with transducer resistance R this resistancebeing identified herein as R,,, the bridge output voltage, E becomes alinear function of reciprocal of Kelvin temperature over a wide range offluid temperatures. Thus, the transducer resistance R and the fixedresistance R,,, arranged in series, comprise one arm of the bridge andthe direct input voltage E is applied thereto. The remaining two bridgeresistors in the four-resistor network are preferably chosen so as tohave a resistance to fix the value of reciprocal of temperature forwhich E O.

The fundamental equation which characterizes the method of the presentinvention is as follows:

ma tn) (RA/RA R1") where R,- is the resistance of the temperaturetransducer or thermometer, R is the resistance of the selected fixedresistor in series with R E is the input voltage applied to the bridgecircuit and E is the output voltage from the bridge circuit.

By a suitable choice of R the following desired relationship isobtained:

where A and B are constants dependent upon the particular temperaturetransducer and T is the absolute temperature of the fluid or othersubstance being tested.

Successful results have been achieved by adapting the method of theinstant invention to an instrument for automatic recording of vaporpressure measurements on synthetic lubricants and hydraulic fluids. Thisinstrument, called a recording tensimeter, supplants prior art methodswhich require laborious manual procedures. The basic design of thetensimeter comprises a sample cell in an air oven connected to apressure transducer outside the oven, with a temperature transducer(having a resistance R located within the oven and near the cell. Thetest fluid being evaluated, of course, is contained in the sample cell.

Ranges of temperature and pressure to be explored with the tensimeterwere to 500C. and to 1,000 torr, respectively. Two separate modes ofoperation were required: (1) recording of vapor pressure throughout thetemperature range with a plot in the form of logarithm of vapor pressureversus reciprocal of temperature; and (2) recording of logarithm ofincrease of vapor pressure in a prescribed time interval at fixedtemperatures in the range, with plotting of logarithm of pressure changeversus reciprocal of temperature. Mode 1) thus represents the simplemeasurement of vapor pressure. Mode (2) provides the measurement ofdecomposition temperature which is defined as the temperature at whichthe pressure exerted by the sample vapor increases at the rate of 50torr per hour due to production of gas by thermal decomposition.

The choice of a logarithm pressure versus reciprocal of Kelvintemperature method of plotting results from the well known relationshipbetween vapor pressure and temperature. A linear relationship is alsotheoretically expected for the logarithm of rate of pressure increase asa function of reciprocal of Kelvin temperature. By implementing themethod of the present invention in design of the automatic tensimeter,therefore, it was possible to use a single x-yrecorder chart to displaythe results of both modes of operation. The motion along one axis of therecorder was made proportional to the reciprocal of Kelvin temperatureand the motion along the other axis was made proportional to thelogarithm of pressure.

It has been found that the method of the present invention is mostadvantageously employed in conjunction with a temperature transducer ofthe platinum resistance transducer type. It has further been found thatthe value of R in series with R in the bridge arm follows a ratherdefined relationship in achieving a bridge output E which is a linearfunction of reciprocal of Kelvin temperature over a wide range oftemperatures. More specifically, it has been found that R is typicallyabout 0.10 toIO'.' 15" timesR at 0C. for platinum resistancetransducers."

To illustrate the method of the present invention for recordingreciprocal of absolute temperature, an automatic recording tensimeter ofthe type described above was employed in making vapor pressure anddecomposition temperature measurementson various liquids of knownproperties. The novel and vital component of the tensimeter was thebridge circuit hereinbefore described. Exemplary compounds tested werehexadecane, dioctyl phthalate and l,3-bis(3-phenoxy) benzene. Ingeneral, the experimental results obtained with the tensimeter were insatisfactory agreement with literature data.

While this invention has been described with respect to various specificexamples and embodiments, it is to be understood that the invention isnot limited thereto and that it can be variously practiced within thescope of the following claims.

The embodiments of this invention in which an exclusive property orprivilege is claimed are as follows:

1. A method for direct recording of a physical property of a substanceas a substantially linear function of the reciprocal of the absolutetemperature of said substance, which comprises the steps of a. exposingsaid substance to a determined temperature;

b. sensing said physical property of said substance and obtaining asignal varying monotonically with the magnitude of said property at saiddetermined temperature;

c. applying said signal to one display input of a readout means;

d. sensing the temperature of said substance with a resistancetemperature transducer;

e. applying the resistance of said transducer to a bridge circuit;

f. applying an input voltage to said bridge circuit;

g. selecting a determined resistance for the series bridge arm of saidbridge circuit with respect to the resistance of said transducer so asto produce an output voltage from said bridge circuit which is asubstantially linear function of the reciprocal of the absolutetemperature of said substance;

h. applying the output voltage from said bridge circuit to anotherdisplay input of said readout means; and

. exposing said substance to further temperatures to obtain indicationson said readout means sufficient to define the relationship between saidphysical property and the reciprocal of the absolute temperature of saidsubstance.

2. A method of claim 1 wherein the is viscosity.

3. A method of claim 1 wherein the is vapor pressure.

4. A method of claim 1 wherein is decomposition rate.

5. A method of claim 1 wherein the resistance which is applied in serieswith the resistance of the transducer is from about 0.10 to about 0.15times the resistance of the transducer at 0C., the transducer being ofthe platinum resistance type.

physical property physical property the physical property i l l

1. A method for direct recording of a physical property of a substanceas a substantially linear function of the reciprocal of the absolutetemperature of said substance, which comprises the steps of a. exposingsaid substance to a determined temperature; b. sensing said physicalproperty of said substance and obtaining a signal varying monotonicallywith the magnitude of said property at said determined temperature; c.applying said signal to one display input of a readout means; d. sensingthe temperature of said substance with a resistance temperaturetransducer; e. applying the resistance of said transducer to a bridgecircuit; f. applying an input voltage to said bridge circuit; g.selecting a determined resistance for the series bridge arm of saidbridge circuit with respect to the resistance of said transducer so asto produce an output voltage from said bridge circuit which is asubstantially linear function of the reciprocal of the absolutetemperature of said substance; h. applying the output voltage from saidbridge circuit to another display input of said readout means; and i.exposing said substance to further temperatures to obtain indications onsaid readout means sufficient to define the relationship between saidphysical property and the reciprocal of the absolute temperature of saidsubstance.
 2. A method of claim 1 wherein the physical property isviscosity.
 3. A method of claim 1 wherein the physical property is vaporpressure.
 4. A method of claim 1 wherein the physical property isdecomposition rate.
 5. A method of claim 1 wherein the resistance whichis applied in series with the resistance of the transducer is from about0.10 to about 0.15 times the resistance of the transducer at 0*C., thetransducer being of the platinum resistance type.